The amount of experience enemies give is determined by their stats (in the "Character" section of the Stats). I'm not very familiar with this aspect, but I believe you need the "Act part" column. E.g. if that column contains 20, I think this means that if a player kills an enemy of Level 11 with these stats, the player will gain 20% of the amount of XP they need to go from level 11 to level 12.
So to change the XP given by enemies in your encounters, you will have to create new stats for them (or override the default stats, but then these will obviously also apply to existing encounters).
I don't think it is possible to create an item that grants a relative XP gain.
